1. Field of the Invention
The present invention concerns improvements in or relating to internpolator and decimator filter structures of the type incorporating recursive digital filters. The invention also concerns the design of notch filters for use in such structures.
2. Related Art
Novel forms of interpolator/decimator filter structure have been described recently in the literature. In particular, reference is made to the two articles: "Digital Signal Processing Schemes for efficient interpolation and Decimation" by R. A. Valanzuala and A. G. Constantinides, reported in IEE. Proc. Vol. 130 No. 6 pp. 225-234 (Dec. 1983); and, "Efficient Sampling Rate alteration using Recursive (IIR) Digital Filters" by R. Ansari and B. Lui, reported in IEEE Trans Acoust., Speech Sig Proc. Vol.ASSP-31 No. 6 pp. 1366-1373 (Dec. 1983).
The structures described are capable of interpolating or decimating between two sampling rates related by a factor N where N is an integer. It proves most advantageous when N is 2, and it is this case that is considered further. It should be noted that Interpolation and Decimation between sampling rates differing by a factor of a power of two is easily implemented by a cascade of similar filters, each changing the frequency by a factor of two.
Important properties of these interpolator and decimator filter structures are as follows:
(i) The repeated use of simple All-Pass-Networks (APNs) to build up the filter structure, facilitating implementation,
(ii) Most of the signal processing is performed at the lower of the two sampling frequencies, reducing the number of multiplications and additions required per unit time to achieve a given performance requirement compared to alternative filter structures,
(iii) Good noise performance,
(iv) Low sensitivity of filter performance to coefficient value, resulting in short wordlength co-efficients,
(v) Reduced number of co-efficients required to implement a filter with given performance compared to alternative conventional filter structures.
However, there are some limitations to the performance of the filter structures as currently described in the literature. In particular such filter structures are incapable of providing more than 3 dB of attenuation at half of the lower sampling frequencies. There are many applications where there is a requirement for more than 3 dB attenuation.